1. Field of the Invention:
The present invention relates to flexographic printing processes. More particularly, the invention relates to a process of printing uncoated paper of stationery grade with an image very similar to that of a wet-laid watermark.
2. Description of the Prior Art:
Watermarking is a process of faintly marking a sheet of paper with a permanent indicia. Such indicia is formed into the paper web during the papermaking process. No inks, colorants or resins are used. Localized orientation of the paper fiber in the indicia pattern provides an opacity difference within the body of the paper sheet.
Traditional watermarking dates back to a time when correspondence stationery was hand formed in single sheets from small batch vessels of aqueous pulp slurry. A forming screen stretched across a framed opening included the watermark pattern as woven into the screen matrix. Since each watermark was a distinctive original and the pulping and forming of paper sheet such a complex process, stationery sheets possessing the watermark carried an association of prestige and security. Counterfeiting a watermark was extremely difficult and rarely attempted.
In similar fashion, watermarks are formed in current, machine laid paper by the use of specialized equipment on the fourdrinier or forming table. In lieu of single sheets of watermarked paper, however, tons of watermarked paper web are produced for subsequent cutting and slitting. Simple economics, therefore, limit the availability of distinctive, individualized, watermarked paper to only the largest stationery users.
To avoid the adverse economics of a genuine, wetlaid, watermark on relatively small quantities of stationery and fine paper, the prior art developed a simulated watermark process whereby colorless solutions of solid resins are printed onto a paper web or sheet. Penetration of the paper by the resin solution alters the paper opacity to translucency. Although barely perceptable when forelighted, the simulated watermark pattern is sharp and distinct when illuminated by backlight.
Unfortunately, prior art resin printed "watermarks" tended to yellow or "age" in a relatively short period of time. Such age yellowing is generally considered unsightly and undesirable.
It is, therefore, an object of the present invention to provide a watermark printing system and formulation that does not yellow with age.
Another object of the present invention is to teach a system for economically printing stationery and other uncoated paper with indicia giving a watermark appearance from either face of a sheet.
Another object of the present invention is to teach a process and formulation for selectively reducing the opacity of uncoated grades of fine or stationery grade paper.